It is already known from U.S. Pat. No. 4,406,308 of Sep. 27, 1983, to vary the beating position of the reed of a terry loom from a "closed" or normal position to an "open" or retracted position for terry formation. The occurs by moving the rocker arm of the reed operating mechanism from its standard position against the action of a return spring, by means of a push rod which is hinged to the shank of the rocker arm. The rocker arm is driven by a control lever which is coupled to the push rod and, by means of a spring, is elastically maintained in contact with the contour of an operating cam driven by the loom drive shaft. To achieve two different terry heights without stopping the loom, two push rods are generally hinged to different points of the rocker arm shank. The selection of which push rod is to shift the rocker arm is determined by two electromagnets rigid with said control lever. The electromagnets are each arranged to magnetically move a pin coupled to the corresponding push rod.
This known device for selecting the rocker arm shift has a series of drawbacks due substantially to the vibration and wear to which the electromagnets used for moving the coupling pins are subjected. Also, the time required by said electromagnets for moving the coupling pins limits their efficiency in modern high-speed terry looms. The electromagnets also utilize considerable energy in moving the pins. A large magnetic field is required to do this with a consequent large power consumption, even if said pins are made with small dimensions to limit their inertia. Finally, the presence of these electromagnets within the region of operation of the coupling pins makes it impossible to seal this region and hence lubricate the pins in an oil bath, as would be required to increase the reliability of the mechanism and to reduce the wear of said pins to a minimum.